A Comparison of Frequency-Dependent Soil Models: Application to the Analysis of Grounding Systems

We present a review and comparison of different models representing the frequency dependence of the soil electrical parameters (conductivity and permittivity). These models are expressed in terms of curve-fit expressions for the soil conductivity and relative permittivity, which are based on experimental data. Six availablemodels/expressions are discussed and compared-making reference to two sets of experimental data. It is shown that the soil models by Scott, Smith-Longmire, Messier, and Visacro-Alipio predict overall similar results, which are in reasonable agreement with both sets of experimental data. Differences between the soil models are found to be more significant at high frequencies and for low-resistivity soils. The causality of the considered models is tested using the Kramers-Kronig relationships. It is shown that the models/expressions of Smith-Longmire, Messier, and Portela satisfy the Kramers-Kronig relationships and thus provide causal results. The soil models are applied to the analysis of grounding systems subject to a lightning current. A full-wave computational model is adopted for the analysis. The analysis is performed considering two cases: 1) a simple horizontal grounding electrode, and 2) a realistic grounding system of a wind turbine. Two current waveforms associated with typical first and subsequent return strokes are adopted for the representation of the incident lightning current. In agreement with recent studies, simulations show that the frequency dependence of the soil parameters results in a decrease of the potential of the grounding electrode, with respect to the case where the parameters are assumed to be constant. It is found that the models/expressions by Scott, Smith-Longmire, Messier, and Visacro-Alipio predict similar levels of decrease, which vary from about 2% (rho(LF) = 20 Omega.m and first stroke) up to 45% (rho(LF) = 10 000 Omega.m and subsequent stroke). On the other hand, the models of Portela and Visacro-Portela predict significantly larger levels of the decrease, especially for very high resistivity soils. Furthermore, in the case of a high resistivity soil (10 000 Omega.m), the Visacro-Alipio expression predicts a longer risetime for the grounding potential rise, compared to the predictions of Scott, Smith-Longmire, and Messier models.

Electromagnetic Environment Associated with Lightning Strikes to Tall Strike Objects

Seyed Abbas Mosaddeghi

The study of the lightning interaction with tall strike objects has attracted considerable attention of lightning researchers lately. Many lightning measurements including current and associated electromagnetic fields were recently made all over the globe namely in Russia, South Africa, Germany, Brazil, Japan, and Austria. It is a novel area of studies, and the resolution of associated questions will have an impact upon many lightning-related applications such as lightning protection and the determination of lightning parameters from remote field measurements. The main objective of the thesis is to carry out further theoretical investigations and experimental measurements to understand and elucidate recently raised questions on the characteristics of lightning return-strokes to tall structures and their associated electromagnetic radiation. Chapter 2 presents a review on recent progress in the modeling of lightning strikes to tall towers and associated experimental data obtained during the last decade or so. Two types of return stroke models namely the Engineering Models, and the Electromagnetic or Antenna-Theory (AT) models, extended to take into account the presence of a tall strike object are discussed. The Chapter contains also a description of the computational methods for the evaluation of electromagnetic fields generated by a lightning strike to a tall structure, as well as an overview of available data on lightning current and associated electromagnetic fields. The chapter finally highlights some important questions raised by different research groups in the past few years which call for further investigations. These questions are as follows: No systematic theoretical analysis nor experimental data are available for electromagnetic fields in the immediate vicinity of a tall structure struck by lightning. The characterization of nearby electromagnetic fields is particularly important in the analysis of the interaction to nearby electrical and electronics systems. Why do lightning return stroke models not reproduce the far-field zero crossing associated with lightning to tall structures? How should these models be revised to be able to reproduce such an effect? How should the engineering models be revised in order to remove the associated current discontinuity at the return stroke wavefront? It is well-known that the measurements of electromagnetic fields from lightning are affected by the presence of nearby buildings and metallic structures. However, no systematic and quantitative analysis of such an effect is presently available in the literature. The work presented in this thesis addresses all of the above questions. The main original contributions of this thesis, consisting of both theoretical and experimental work, are presented in Chapters 3 through 6. Chapter 3 is devoted to a theoretical description of the signature of electric and magnetic fields at very close distance associated with lightning strikes to a tower. It is shown that the electric field generated by a lightning return stroke to a tall structure can change polarity at very close distance range. This change in the polarity seems to be a specific signature of the very close vertical electric field. A simple equation is derived which provides an estimate of the critical distance below which such an inversion of polarity might occur. It is also shown that the inversion of polarity depends on the value of the reflection coefficient at the base of the tower and disappears for reflection coefficients close to 1. On the other hand, other parameters such as the return stroke speed, the reflection coefficient at the top of the strike object, and the adopted return stroke model seem not to have an impact on the inversion of polarity. Simulation results also showed that the electric field peak at distances beyond the height of the tower or so exhibits the typical 1/r dependence. At closer distances, however, the E-field peak features a saturation, due to the so-called tower shadowing effect. This shadowing effect results in a substantial decrease of the nearby electric field. On the other hand, the magnetic field peak varies inversely proportional to the horizontal distance and does not depend significantly on the presence of an elevated strike object. Chapter 4 introduces an improved version of the engineering models for return-strokes to tall structures which accounts for (1) the presence of possible reflections at the return stroke wavefront, and, (2) a return stroke initiation above the structure due to an upward connecting leader. We also propose an elegant iterative solution that can be easily implemented into computer simulation programs to take into account in a straightforward way multiple reflections occurring at the discontinuities at the tower ends and at the return stroke wavefront. Simulation results for the magnetic fields are compared with experimental waveforms associated with lightning strikes to the CN Tower (553 m). It is shown that taking into account the reflections at the return-stroke wavefront results in better reproducing the fine structure of the magnetic field waveforms. Chapter 5 presents and discusses obtained measurements of electric (vertical and radial) and magnetic fields from leaders and return strokes associated with lightning strikes to the Gaisberg tower in Austria obtained in 2007 and 2008. The data include simultaneous records of vertical and radial electric fields, which were obtained for the first time at such close distances. It is found that the vertical and radial electric field waveforms appear as asymmetrical V-shaped pulses. For the vertical electric field, the initial, relatively slow, negative electric field change is due to the downward leader and the following fast positive field change is due to the upward return stroke phase of the lightning discharge. For the horizontal electric fields, however, the bottom of the V is not associated with the transition from the leader to the return stroke. The horizontal field change due to the return stroke is characterized by a short negative pulse of the order of one microsecond or so, starting with a fast negative excursion followed by a positive one. In addition, an analytical expression for the radial electric field, assuming a uniform charge distribution along the leader with constant speed is derived. It is also shown that the return-stroke vertical electric field changes appear to be significantly smaller than similar measurements obtained using triggered lightning. This finding confirms the shadowing effect of the tower predicted by the theoretical analysis of Chapter 3, which results in a significant decrease of the electric field at distances of about the height of the tower or less. Finally, the ability of two different models for the return stroke in reproducing measured vertical and horizontal electric fields is tested using the obtained measured data. The considered models are (1) the engineering MTLE (Modified Transmission Line with Exponential Decay) model, and (2) the electromagnetic model implemented using the Numerical Electromagnetics Code NEC-4. It is shown that both models predict electric field waveforms which are in reasonable agreement with measured waveforms. In general, the predicted fields by the electromagnetic model appear to be in better agreement with measured data, because of the direct use of the measured current waveform as an input and the more accurate representation of the tower. Chapter 6 reports on the effect of nearby buildings on electromagnetic fields from lightning. Indeed, sensors used for the measurement of lightning electric and magnetic fields are often placed close to or on top of buildings or other structures. Metallic beams and other conducting parts in those structures may cause enhancement or attenuation effects on the measured fields. Experimental waveforms radiated from distant natural lightning recorded during the summers of 2006 and 2007 are presented. Electric and magnetic field waveforms were measured simultaneously on the roof of a building and on the ground at different distances away from it. The results suggest that the measured electric field on the roof of the building could be enhanced by a factor of 1.7 to 1.9, whereas the electric fields on the ground experienced a significant reduction by a factor ranging from 5 to 20. Also, it is shown that for a sensor located on the ground close to a building, the magnetic field component perpendicular to the building can experience significant attenuation, presumably due to the effect of the induced currents in the building. The magnetic field on the roof of the building seems not to be significantly affected by the building. Simulations using the Numerical Electromagnetic Code (NEC-4) were also carried out in which the building was represented using a simple wire-grid model. The simulation results support in essence the findings of the experimental analysis, despite quantitative differences which are ascribed, at least in part, to the oversimplified model of the building.

EPFL2011

Advanced methods for the evaluation of lightning incidence over large areas, wind turbine parks and tall towers

Aleksandr Smorgonskii

Lightning presents a significant danger to electrical and electronic equipment, buildings and monuments, people and animals. Although our knowledge about lightning has significantly improved over last century, the development of new composite materials and the construction of an increasing number of tall structures present new challenges to the lightning protection techniques. Electric power generation based on wind energy is an extremely fast growing sector of renewable energy production. Lightning is the major cause of damage to wind turbines and their components. While modern wind turbines measure 150 m in height, future wind power generation units will be characterized by even taller turbines, as a result, it is expected that wind turbines will be more exposed to lightning strikes. Within this context, three key problems related to lightning protection of wind turbines and other tall structures were identified and studied within this thesis. First, the problem of the initiation of upward flashes from tall structures is addressed. Two techniques are commonly used to study this process in real scale: direct measurements of lightning currents using tall towers and rocket-triggered lightning experiments. An advanced analytical model was developed to study the relation between the parameters of ground and wire corona during triggered lightning experiments. The influence of several previously disregarded parameters was studied, which is a significant improvement over formerly used methods. An analysis of the initiation of upward lightning flashes for the Gaisberg and the Säntis Towers is presented that is based on the data from the European lightning location system (EUCLID) and direct lightning current measurements at the towers. Secondly, a new method for the estimation of the number of upward flashes from a tall structure is developed. The method is based on the analysis of the data provided by lightning location systems and it can therefore be applied to any tall tower or wind turbine covered by a lightning location system. Unlike available methods, it can be applied for structures in various locations with complex terrain. The method is applied to various structures around Europe. It is also shown that the method used in the current standards to esti-mate the risks due to lightning can result in underestimated values of lightning incidence and therefore in the choice of an inade-quate lightning protection level. Using the proposed approach, the proportion of upward flashes observed at different research towers used for lightning measure-ments is analysed. The obtained results confirm the validity of the parameters of downward flashes obtained at this tower and used in current standards on lightning protection. The application of the proposed method to evaluate lightning incidence to a group of tall structures is also presented in the thesis. The case of the Mont-Crosin wind turbine park was used to illustrate it, for which the lightning incidence is evaluated over several time periods corresponding to the construction periods of the wind turbine park. Finally, the last question discussed in this thesis is the distribution of lightning flashes over large areas. Specifically, the distribution of lightning flash density as a function of terrain elevation is analysed. Two improved methods were proposed. The lightning flash density distributions over the Alps and the Pyrenees are then evaluated using the proposed methods and the obtained results suggest that the lightning flash density grows over the whole altitude range.

EPFL2016

Isolated vs. Interconnected Wind Turbine Grounding Systems: Effect on the Harmonic Grounding Impedance, Ground Potential Rise and Step Voltage

Qi Li, Marcos Rubinstein, Antonio Sunjerga

Wind turbines are very vulnerable to lightning strikes due to their height, sharp edges and remote locations often with high soil resistivity. In this paper we present numerical simulations of the impedance of a typical wind turbine grounding geometry. We analyze the influence of interconnecting grounding systems of different wind turbines. IEC TR61400-24 suggests interconnection of grounding electrodes of wind turbines through horizontal electrodes (in the form of insulated or bare conductors) to achieve low steady-state grounding resistance. The analysis takes into account the frequency dependence of the soil electrical parameters. We show that the low frequency grounding impedance can be reduced by a factor of two or more as a result of interconnecting grounding systems. However, the reduction is significantly lower at higher frequencies because of the interconnection wire’s inductance. We analyze the spatial distribution of the ground potential rise and step voltage in response to typical first and subsequent lightning return stroke current waveforms. It is shown that both, ground potential rise and step voltage can be significant along the wire, especially for high resistivity soil, and placing sensitive equipment near the interconnecting wire should be either avoided, or insulated wire should be used.